In addition to "Lin's" post below, you may also consider looking at the first 3 or 4 posts in this thread: http://www.diytube.com/phpBB2/viewtopic.php?p=23087 which say that while usable, for linearity, noise and microphonic reasons the 6BQ7A is not a better replacement for 6NP1-EV/6922 type tubes. It appears this type of tube is mostly used in the front end of FM Tuners. That said, on Decware Forums "Rivieraranch" likes the 6BQ7A and 6N1P and "Lon" prefers the 6n1p (see "Lin's" post below).

However, notwithstanding the desire to experiment, which many of us have, Steve's recommendation for the input positions of the SE34I.3 "Rachael" or CSP+, are either the 6N1P-EV or 6DJ8 family of tubes (6922, E88CC, 7308, E188CC, CCa, 6H23, PCC88, 7DJ8).